Method for bleaching wood fibers

ABSTRACT

A method for bleaching wood fibers with a mixture of sodium borohydride and sodium bisulfite followed by peroxide bleaching in the same vessel.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This is a non-provisional application of prior pending U.S. Provisional Application Ser. No. 60/619,154 filed on Oct. 15, 2004.

BACKGROUND

This invention relates generally to a method for bleaching wood fibers with mixture of sodium borohydride and sodium bisulfite followed by peroxide bleaching.

Combinations of reductive and oxidative bleaching have been used to bleach wood fibers. For example, WO 01/59205 discloses a method for bleaching mechanical and chemithermomechanical pulp by pre-bleaching the pulp with a reductive bleaching agent, followed by peroxide bleaching reinforced with borohydride. The reductive agents mentioned in this reference are hydrosulfite, borohydride, hydrazine and formamidine sulfinic acid (FAS). However, there are hazards associated with handling these reductive agents, alone and/or in combination with peroxide.

The problem addressed by this invention is to find a safer and more effective process for reductive bleaching followed by peroxide bleaching.

STATEMENT OF THE INVENTION

This invention is directed to a method for bleaching wood fibers by pre-treating the fibers with a mixture of sodium borohydride and sodium bisulfite, followed by peroxide bleaching.

The method comprises the steps of:

(a) adding to a vessel containing a wood fiber slurry a mixture produced by combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite, to produce a treated slurry; and (b) adding an aqueous solution comprising hydrogen peroxide to the treated slurry in said vessel.

DETAILED DESCRIPTION OF THE INVENTION

All percentages are expressed as weight percentages, unless specified otherwise. The term “Pre-mix” (PMX) refers to a process in which borohydride and bisulfite are mixed prior to addition to the wood fibers.

Dithionite ion, also referred to as hydrosulfite, can be produced by the reaction between bisulfite and borohydride ions, according to the following theoretical equation: BH₄ ⁻+8HSO₃ ⁻+H⁺→4S₂O₄ ⁻²+B(OH)₃+5H₂O The yield is somewhat less than 100% due to competing reactions, including that of borohydride with water, but is most often better than 85%. Since the exact mechanism of the reaction has not been fully characterized, this invention is not limited to reduction by dithionite ion, and other species present in the reaction mixture also may act as reducing agents. When the amount of bisulfite is at 8 moles per mole of borohydride, the theoretical reaction proceeds to completion. Without wishing to be bound by theory, it is believed that use of more than the theoretical amount of bisulfite or less than the theoretical amount of bisulfite results in a mixture containing hydrosulfite, sodium bisulfite, borohydride and possibly other species.

In a preferred embodiment of the invention, in which borohydride is mixed with aqueous sodium bisulfite solution in the form of an aqueous solution containing sodium borohydride and sodium hydroxide, some of the bisulfite is consumed in a neutralization reaction with the hydroxide ion. In some applications, hydroxide ion present in borohydride solutions is neutralized by acid added to the bisulfite solution. In such a case, to the extent that the hydroxide initially present in the borohydride solution has been neutralized, it will not consume bisulfite, and will not be included in the ratio calculation. As described above, the theoretical reaction of borohydride and bisulfite requires 8 moles of unconsumed bisulfite per mole of borohydride, i.e., the ratio (moles bisulfite-moles hydroxide)/moles borohydride is approximately 8. The present invention uses a ratio from 1 to 25. Preferably, the ratio is no more than 21, more preferably no more than 18, and most preferably no more than 16. Preferably, the ratio is at least 3, more preferably at least 5, and most preferably at least 6.

In the embodiment of the invention, in which the fiber slurry is pre-treated with borohydride and bisulfite solutions which are combined and then added to the slurry, the ratio (moles bisulfite-moles hydroxide)/moles borohydride combined for addition to the pulp (i.e., excluding the bisulfite used for pre-treatment) preferably is at least 3, more preferably at least 5, and most preferably at least 6; preferably the ratio is no more than 21, more preferably no more than 18, more preferably no more than 16, more preferably no more than 14, and most preferably no more than 10. The time interval between addition of the pre-mixed solutions and addition of chemicals used in peroxide stage is not believed to be critical. Preferably, a time interval sufficient to ensure thorough mixing of the pre-mixed solution and the pulp is allowed, preferably at least 5 seconds, more preferably at least 20 seconds, and most preferably at least 30 seconds. Preferably, the time interval is no longer than 2 hours, more preferably no longer than 1 hour, more preferably no longer than 30 minutes, and most preferably no longer than 15 minutes.

In one embodiment of the invention, combining water and sodium metabisulfite, Na₂S₂O₅, generates bisulfite. The aqueous sodium bisulfite preferably is about 2% to about 45% active by weight. A preferred borohydride composition for use in accordance with the methods of the invention is in liquid form and comprises about 1% to about 36% active sodium borohydride and about 30 to about 40% NaOH or Na₂CO₃ (also known as soda ash), all by weight. A particularly preferred borohydride composition comprises approximately 12% active sodium borohydride and 40% NaOH.

In one embodiment of this invention, preferably, the addition of pre-mixed solution for pretreatment is carried out by adding pre-mixed solution to the pulp slurry after the slurry has been screened, cleaned, thickened and made ready for papermaking i.e., after the deckers in a typical pulp mill. In one preferred embodiment of the invention, the pre-mixed solution is added to the disperser followed by adding chemicals for the peroxide stage at the conveying screw that leads to a down-flow tower or chest.

The method of this invention is a single-stage bleaching process, i.e., a process in which the reductive and oxidative bleaching are done in the same vessel, without a washing or contamination removal step between the reductive and oxidative bleaching. In contrast, two-stage bleaching typically involves sending the pulp slurry to another vessel for washing, pressing, or another purification step prior to another bleaching step, e.g., oxidative bleaching.

In the embodiments of this invention, a borohydride solution and a bisulfite solution are combined to produce a mixture which is then added to the pulp slurry. Preferably, the solutions are combined by mixing them in an in-line static mixer, a T-pipe mixer, a pump recirculator, an agitator, the suction side of a pump impeller, a high-shear mixer, or a low-shear mixer. Most preferably, the mixer is an in-line static mixer. Typical in-line static mixers have from 2 to 24 internal elements, preferably from 2 to 6 internal elements. The length of the piping from the mixer to the point of addition to the pulp slurry also may affect the mixing; preferably this length is at least 3.28 ft (1 m), more preferably at least 4.92 ft (1.5 m). The number of elements, the diameter of the mixer and the length of piping required to achieve good mixing, i.e., to produce a substantially homogeneous mixture, can be determined easily from the flow parameters and fluid properties of each particular system. For example, in one method dye is added to one of the solutions and good mixing is assessed by visible determination that the color of the output is uniform. In another method, the pH of the pulp slurry after addition of the mixed borohydride and bisulfite solutions is measured; a stable pH value is an indication of good mixing, as are consistent bleaching results. Preferably, if mixing is insufficient, the borohydride and bisulfite solutions are diluted. Preferably, the borohydride and bisulfite solutions are mixed at a temperature in the range from 4° C. (39.2° F.) to 50° C. (122° F.), more preferably from 10° C. (50° F.) to 35° C. (95° F.).

Preferably, the mixed borohydride and bisulfite solutions are added to the pulp slurry directly, or by storing the output in a vessel for later addition to the pulp slurry. In one preferred embodiment, the output of the mixer is stored in a vessel and added to the pulp slurry within 12 hours of mixing, more preferably-within 6 hours, more preferably within 3 hours, more preferably within 1 hour, and most preferably within ½ hour of mixing. In another preferred embodiment, the mixer output is added directly through piping which carries the output to the pulp slurry in less than 15 minutes, more preferably less than 10 minutes, and most preferably less than 5 minutes.

Preferably, the amount of borohydride combined with sodium bisulfite, measured as the percentage of sodium borohydride relative to the dried fiber content of the pulp, is at least 0.0006%, more preferably at least 0.0009%, and most preferably at least 0.0015%. Preferably, the amount of borohydride, measured as the percentage of sodium borohydride relative to the dried fiber content of the pulp, is no more than 0.12%, more preferably no more than 0.08%, more preferably no more than 0.045%, and most preferably no more than 0.03%. In a preferred embodiment of the invention, a 12% aqueous sodium borohydride solution stabilized with 40% NaOH is used. In this embodiment, the weight of the solution used, measured as a percentage of the dried fiber content of the pulp, is at least 0.005%, more preferably at least 0.0075%, and most preferably at least 0.0125%. Preferably, the weight of solution used, measured as a percentage of the dried fiber content of the pulp, is no more than 1%, more preferably no more than 0.67%, and most preferably no more than 0.25%.

Preferably, the amount of sodium bisulfite combined with borohydride, measured as the percentage of sodium bisulfite relative to the dried fiber content of the pulp, is at least 0.02%, more preferably at least 0.03%, and most preferably at least 0.05%; preferably the total amount is no more than 4%, more preferably no more than 2.8%, more preferably no more than 1.5% and most preferably no more than 1%.

The peroxide bleaching stage can carried out conventionally. The preferred peroxide agent is hydrogen peroxide. Preferably, hydrogen peroxide is provided in the form of an aqueous solution, most preferably approximately 50% aqueous hydrogen peroxide. The initial aqueous hydrogen peroxide solution may be diluted with water to facilitate addition to the pulp slurry. The amount of hydrogen peroxide measured as the percentage of pure hydrogen peroxide relative to the dried fiber content of the pulp is 0.5 to 5%. Preferably, the bleaching time is 30 minutes to 4 hours. More preferably, the bleaching time is at least 1 hour, and no more than 2 hours. Other peroxide enhancement agents can also be added with peroxide. Examples of such agents are sodium hydroxide (1 to 2.5%), sodium silicate (1 to 5%), and chelating agents, e.g., DTPA (diethylenetriamine pentaacetic acid) and EDTA (ethylene diamine tetraacetic acid, at 0.1 to 0.5%.

Preferably, the wood fibers used in this invention are fibers derived from recycled paper, mechanical pulp or chemical pulp. Most preferably, the fibers are derived from recycled paper. The initial (prior to bleaching) consistency of the pulp slurry preferably is from 2% to 50%, and more preferably from 8% to 30%. In general, the initial pH of the pulp slurry preferably is from 2 to 13. For mechanical pulp, the pH preferably is from 4 to 8; for chemical pulp, from 3 to 12, more preferably from 7 to 12; and for recycled fibers, from 5 to 9. The temperature for the bleaching stages preferably is from 20° C. to 120° C., more preferably from 50° C. to 90° C.

EXAMPLES

Table 1 shows the results of a comparison between (PMX)-P (pre-mix followed by peroxide bleaching) and FAS-P processes on a mixed office waste furnish from mill 1. Pre-mix solution was generated using a (bisulfite-NaOH) to borohydride molar ratio of 8.9:1. Reductive pretreatments were conducted at various amounts of Pre-mix and FAS, followed immediately after thorough mixing of PMX or FAS into the pulp (ca. 1 minute) by peroxide addition to the pre-treated pulp. Bleaching conditions are listed in Table 1. It is clear from the table that PMX-P process performed significantly better than FAS-P process. Table 2 shows the results of a comparison among (PMX)-P, FAS-P, and Y-P processes on a recycled fiber furnish from mill 2. Bleaching was conducted in the same way as for mill 1 pulp. Bleaching conditions can be seen in Table 2. The results on this pulp also demonstrate that (PMX)-P process performs significantly better than FAS-P and Y-P processes. TABLE 1 Laboratory bleaching response of (PMX)-P and (FAS)-P processes on pulp sample from mill 1. Process FAS % NaBH4 % Bisulfite % pH final Bright. % ISO L* a* b* Unbleached 0 0 0 — 64.9 86.9 0.37 4.33 FAS-P 0.125 10.02 74.7 91.1 0.82 3.12 0.25 10.07 74.6 91.3 0.43 3.58 0.375 10.11 75.1 91.6 0.17 3.72 0.5 10.16 75.5 91.8 0.15 3.66 (PMX)-P 0.0045 0.15 9.98 75.6 91.4 0.60 2.93 0.009 0.3 9.94 76.3 91.7 0.45 2.89 0.0135 0.45 9.87 75.8 91.5 0.49 2.94 0.018 0.6 9.83 75.9 91.6 0.47 2.97 Notes: The initial pH of the pulp slurry was 7.71 in all cases. The unbleached pulp sample had a consistency of 25.96%. Bleaching conditions: 77° C., 10% consistency, 1 minute for FAS and pre-mix stages, 60 minutes for P stage. FAS stage: commercial product, caustic to FAS weight ratio 1:1. Peroxide stage: 1.75% peroxide, 0.75% caustic, 1% silicate (as 27% SiO₂ solution). “PMX” means mixture solution of borohydride and bisulfite. The molar ratio of (bisulfite - NaOH) to borohydride is 8.9:1.

TABLE 2 Laboratory bleaching response of (PMX)-P, (FAS)-P, and Y-P processes on pulp sample from mill 2. Process FAS/Y % NaBH₄ % Bisulfite % pH final Bright. % ISO L* a* b* Unbleached 0 0 0 — 71.6 89.4 0.80 2.75 (FAS)-P 0.1 10.02 79.3 92.1 1.51 1.31 0.2 79.4 92.3 1.44 1.46 0.3 10.07 79.6 92.6 0.99 2.0 0.4 10.11 80.5 93.3 0.61 2.34 0.5 10.16 80.8 93.4 0.39 2.43 Y-P 0.05 9.98 79.4 92.4 1.11 1.53 0.1 80.0 92.7 0.96 1.56 0.15 9.94 80.4 92.9 0.82 1.65 0.2 9.87 80.5 92.9 0.79 1.64 0.3 9.83 80.8 93.1 0.68 1.73 (PMX)-P 0.003 0.1 9.98 81.0 92.9 1.38 1.42 0.006 0.2 81.7 93.4 1.06 1.65 0.009 0.3 9.94 82.0 93.5 0.99 1.60 0.012 0.4 9.87 81.8 93.4 0.98 1.65 0.018 0.6 9.83 81.7 93.5 0.86 1.84 Notes: The initial pH of the pulp slurry was 8.15 in all cases. The unbleached pulp sample had a consistency of 32.21%. Bleaching conditions: 69° C. (155° F.), 10% consistency, 1 minute for FAS, BGH, and PMX stage, 60 minutes for P stage. FAS stage: commercial product, caustic to FAS weight ratio 1:1. Peroxide stage: 1.5% peroxide, 1% caustic, 1% silicate (as 27% SiO₂ solution). “PMX” means a mixture of solutions of borohydride and bisulfite. The molar ratio of (bisulfite - NaOH) to borohydride was 8.9:1. “Y” means sodium hydrosulfite generated by 1 mole of 12% SBH & 40% NaOH solution, 8 moles of sodium bisulfite, and 1.7 moles of sulfuric acid. The pH of hydrosulfite solution is 6.8. Laboratory Bleaching Procedures

Based on consistency, 7 g O.D. pulp was placed in heavy gauge polyethylene bags. The bags were sealed under nitrogen, shaken vigorously to disperse the pulp fiber, and preheated in a constant temperature bath to get to the desired bleach temperature. Pre-mix solutions were generated from an aqueous solution containing 12% sodium borohydride and 40% sodium hydroxide, and NaHSO₃ (SBS). In the method of generating the pre-mix solution, sodium bisulfite powder was added to water in a round bottom flask and stirred until the sodium bisulfite powder had completely dissolved. The sodium borohydride solution was then immediately added under an inert atmosphere and under rapid stirring to generate a completely formed pre-mix solution. Based on the borohydride concentration of each solution, the required volume of pre-mix solutions was calculated. Bleaching was carried out by adding the pre-mix solution or FAS solution combined with caustic or hydrosulfite solution under nitrogen and keeping the pipette below the surface of the pulp. Each bag was resealed, kneaded thoroughly to mix for 1 minute. Peroxide together with caustic and silicate were then added to the pulp and kneaded thoroughly to mix and returned to the constant temperature bath for the desired time. At the end of the bleaching period each bag was removed from the bath and the pH was measured and recorded. The pulp was then diluted to 1% using deionized water and dewatered using a 100-mess screen. The pulp was diluted to 1% consistency again with deionized water and the brightness pad was made using a Buchner funnel. One pad was made from each run and air dried overnight at 50% relative humidity. Brightness readings were measured using a Technibrite™ ERIC 950 and are the average of five readings from each brightness pad. 

1. A method for bleaching wood fibers; said method comprising steps of: (a) adding to a vessel containing a wood fiber slurry a mixture produced by combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite, to produce a treated slurry; and (b) adding an aqueous solution comprising hydrogen peroxide to the treated slurry in said vessel.
 2. The method of claim 1 in which the aqueous solution comprising hydrogen peroxide is added from 5 seconds to 2 hours after said mixture.
 3. The method of claim 2 in which an amount of sodium borohydride used to produce said mixture is from 0.0006% to 0.045%, based on the weight of dry fiber.
 4. The method of claim 3 in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 1 to
 25. 5. The method of claim 4 in which an amount of sodium bisulfite added is from 0.02% to 1.5%.
 6. The method of claim 5 in which the wood fiber slurry is derived from recycled paper.
 7. The method of claim 6 in which the aqueous solution comprising hydrogen peroxide is added from 5 seconds to 15 minutes after said mixture.
 8. The method of claim 7 in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 6 to
 10. 